Cryptography and
Network Security
Chapter 3
Fifth Edition
by William Stallings
Lecture slides by Lawrie Brown
Chapter 3 – Block Ciphers and
the Data Encryption Standard
All the afternoon Mungo had been working on
Stern's code, principally with the aid of the latest
messages which he had copied down at the
Nevin Square drop. Stern was very confident.
He must be well aware London Central knew
about that drop. It was obvious that they didn't
care how often Mungo read their messages, so
confident were they in the impenetrability of the
code.
—Talking to Strange Men, Ruth Rendell
2
Modern Block Ciphers
 now
look at modern block ciphers
 one of the most widely used types of
cryptographic algorithms
 provide secrecy /authentication services
 focus on DES (Data Encryption Standard)
 to illustrate block cipher design principles
3
Block vs Stream Ciphers
 block
ciphers process messages in blocks,
each of which is then en/decrypted
 like a substitution on very big characters
 64-bits
or more
 stream
ciphers process messages a bit or
byte at a time when en/decrypting
 many current ciphers are block ciphers
 better
analysed
 broader range of applications
4
Block vs Stream Ciphers
5
Block Cipher Principles






most symmetric block ciphers are based on a
Feistel Cipher Structure
needed since must be able to decrypt ciphertext
to recover messages efficiently
block ciphers look like an extremely large
substitution
would need table of 264 entries for a 64-bit block
instead create from smaller building blocks
using idea of a product cipher
6
Ideal Block Cipher
7
Claude Shannon and SubstitutionPermutation Ciphers

Claude Shannon introduced idea of substitutionpermutation (S-P) networks in 1949 paper
 form basis of modern block ciphers
 S-P nets are based on the two primitive
cryptographic operations seen before:



substitution (S-box)
permutation (P-box)
provide confusion & diffusion of message & key
8
Confusion and Diffusion
 cipher
needs to completely obscure
statistical properties of original message
 a one-time pad does this
 more practically Shannon suggested
combining S & P elements to obtain:
 diffusion – dissipate statistical structure of
plaintext over bulk of ciphertext
 confusion – makes relationship between
ciphertext and key as complex as possible
9
Feistel Cipher Structure
 Horst
Feistel devised the feistel cipher
 based
on concept of invertible product cipher
 partitions
input block into two halves
 process
through multiple rounds which
 perform a substitution on left data half
 based on round function of right half & subkey
 then have permutation swapping halves
 implements
Shannon’s S-P net concept
10
Feistel Cipher Structure
11
Feistel Cipher Design Elements
 block
size
 key size
 number of rounds
 subkey generation algorithm
 round function
 fast software en/decryption
 ease of analysis
16
FEISTEL DECRYPTION
ALGORITHM





The process of decryption with a Feistel cipher is
essentially the same as the encryption process.
The rule is as follows:
Use the ciphertext as input to the algorithm, but
use the subkeys Ki in reverse order.
That is, use Kn in the first round, Kn-1in the
second round, and so on, until is used in the
last round
Data Encryption Standard (DES)
 most
widely used block cipher in world
 adopted in 1977 by NBS (now NIST)
 as
FIPS PUB 46
 encrypts
64-bit data using 56-bit key
 has widespread use
 has been considerable controversy over
its security
24
DES History
 IBM
developed Lucifer cipher
 by
team led by Feistel in late 60’s
 used 64-bit data blocks with 128-bit key
 then
redeveloped as a commercial cipher
with input from NSA and others
 in 1973 NBS issued request for proposals
for a national cipher standard
 IBM submitted their revised Lucifer which
was eventually accepted as the DES
25
DES Design Controversy
 although
DES standard is public
 was considerable controversy over design
 in
choice of 56-bit key (vs Lucifer 128-bit)
 and because design criteria were classified
 subsequent
events and public analysis
show in fact design was appropriate
 use of DES has flourished
 especially
in financial applications
 still standardised for legacy application use
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DES Encryption Overview
27
Initial Permutation IP
 first
step of the data computation
 IP reorders the input data bits
 even bits to LH half, odd bits to RH half
 quite regular in structure (easy in h/w)
 example:
IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb)
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bbb
bbbbbbb
DES Round Structure
 uses
two 32-bit L & R halves
 as for any Feistel cipher can describe as:
Li = Ri–1
Ri = Li–1  F(Ri–1, Ki)
F
takes 32-bit R half and 48-bit subkey:
 expands
R to 48-bits using perm E
 adds to subkey using XOR
 passes through 8 S-boxes to get 32-bit result
 finally permutes using 32-bit perm P
33
b
Substitution Boxes S
 have
eight S-boxes which map 6 to 4 bits
 each S-box is actually 4 little 4 bit boxes
 outer
bits 1 & 6 (row bits) select one row of 4
 inner bits 2-5 (col bits) are substituted
 result is 8 lots of 4 bits, or 32 bits
 row
selection depends on both data & key
 feature
known as autoclaving (autokeying)
 example:

S(18 09 12 3d 11 17 38 39) = 5fd25e03
37
DES Round Structure
38
DES Key Schedule
 forms
subkeys used in each round
 initial
permutation of the key (PC1) which
selects 56-bits in two 28-bit halves
 16 stages consisting of:
 rotating
each half separately either 1 or 2 places
depending on the key rotation schedule K
 selecting 24-bits from each half & permuting them
by PC2 for use in round function F
 note
practical use issues in h/w vs s/w
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DES Decryption

decrypt must unwind steps of data computation
 with Feistel design, do encryption steps again
using subkeys in reverse order (SK16 … SK1)

IP undoes final FP step of encryption
 1st round with SK16 undoes 16th encrypt round
 ….
 16th round with SK1 undoes 1st encrypt round
 then final FP undoes initial encryption IP
 thus recovering original data value
40
DES Example
41
Avalanche in DES
42
Avalanche Effect
 key
desirable property of encryption alg
 where a change of one input or key bit
results in changing approx half output bits
 making attempts to “home-in” by guessing
keys impossible
 DES exhibits strong avalanche
43
Strength of DES – Key Size
keys have 256 = 7.2 x 1016 values
 brute force search looks hard
 recent advances have shown is possible
 56-bit
 in
1997 on Internet in a few months
 in 1998 on dedicated h/w (EFF) in a few days
 in 1999 above combined in 22hrs!
 still
must be able to recognize plaintext
 must now consider alternatives to DES
44

Strength- The strength of DES lies on two facts:

The use of 56-bit keys: 56-bit key is used in
encryption, there are 256 possible keys. A brute
force attack on such number of keys is impractical.
 The nature of algorithm: Cryptanalyst can perform
cryptanalysis by exploiting the characteristic of DES
algorithm but no one has succeeded in finding out
the weakness.
 Weakness- Weakness has been found in the
design of the cipher:

Two chosen input to an S-box can create the same
output.
 The purpose of initial and final permutation is not
clear.
Strength of DES – Analytic
Attacks

now have several analytic attacks on DES
 these utilise some deep structure of the cipher

by gathering information about encryptions
 can eventually recover some/all of the sub-key bits
 if necessary then exhaustively search for the rest

generally these are statistical attacks

differential cryptanalysis
 linear cryptanalysis
 related key attacks
46
Strength of DES – Timing
Attacks
 attacks
actual implementation of cipher
 use knowledge of consequences of
implementation to derive information about
some/all subkey bits
 specifically use fact that calculations can
take varying times depending on the value
of the inputs to it
 particularly problematic on smartcards
47
Differential Cryptanalysis
 one
of the most significant recent (public)
advances in cryptanalysis
 known by NSA in 70's cf DES design
 Murphy, Biham & Shamir published in 90’s
 powerful method to analyse block ciphers
 used to analyse most current block ciphers
with varying degrees of success
 DES reasonably resistant to it, cf Lucifer
48
Differential Cryptanalysis
a
statistical attack against Feistel ciphers
 uses cipher structure not previously used
 design of S-P networks has output of
function f influenced by both input & key
 hence cannot trace values back through
cipher without knowing value of the key
 differential cryptanalysis compares two
related pairs of encryptions
49
Differential Cryptanalysis
Compares Pairs of Encryptions
 with
a known difference in the input
 searching for a known difference in output
 when same subkeys are used
50
Differential Cryptanalysis
 have
some input difference giving some
output difference with probability p
 if find instances of some higher probability
input / output difference pairs occurring
 can infer subkey that was used in round
 then must iterate process over many
rounds (with decreasing probabilities)
51
Differential Cryptanalysis
52
Differential Cryptanalysis


perform attack by repeatedly encrypting plaintext pairs
with known input XOR until obtain desired output XOR
when found



can then deduce keys values for the rounds




if intermediate rounds match required XOR have a right pair
if not then have a wrong pair, relative ratio is S/N for attack
right pairs suggest same key bits
wrong pairs give random values
for large numbers of rounds, probability is so low that
more pairs are required than exist with 64-bit inputs
Biham and Shamir have shown how a 13-round iterated
characteristic can break the full 16-round DES
53
Linear Cryptanalysis
 another
recent development
 also a statistical method
 must be iterated over rounds, with
decreasing probabilities
 developed by Matsui et al in early 90's
 based on finding linear approximations
 can attack DES with 243 known plaintexts,
easier but still in practise infeasible
54
Linear Cryptanalysis
 find
linear approximations with prob p != ½
P[i1,i2,...,ia]
C[j1,j2,...,jb] =
K[k1,k2,...,kc]
where ia,jb,kc are bit locations in P,C,K
 gives
linear equation for key bits
 get one key bit using max likelihood alg
 using a large number of trial encryptions
 effectiveness given by: |p–1/2|
55
DES Design Criteria
 as
reported by Coppersmith in [COPP94]
 7 criteria for S-boxes provide for
 non-linearity
 resistance
to differential cryptanalysis
 good confusion
3
criteria for permutation P provide for
 increased
diffusion
56
Block Cipher Design
 basic
principles still like Feistel’s in 1970’s
 number of rounds
 more
is better, exhaustive search best attack
 function
f:
 provides
“confusion”, is nonlinear, avalanche
 have issues of how S-boxes are selected
 key
schedule
 complex
subkey creation, key avalanche
57
Summary
 have
considered:
 block
vs stream ciphers
 Feistel cipher design & structure
 DES
 details
 strength
 Differential
& Linear Cryptanalysis
 block cipher design principles
58